modern genetics
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Modern Genetics. Part 5. History. Gregor Mendel : “Father of Genetics” Austrian Monk (1822-1884) Published the results of scientific research on Garden Peas ( Pisium sativum ) in 1865. Why study peas? - PowerPoint PPT PresentationTRANSCRIPT
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Modern GeneticsPart 5
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Gregor Mendel: “Father of Genetics” Austrian Monk (1822-1884) Published the results of scientific research on
Garden Peas (Pisium sativum) in 1865.
Why study peas? Great natural variation- stem length, seed
color, pod shape ,pod color, small, edible, easy to grow, many offspring, easy to cross fertilize
History
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Trait: characteristic of an organism. Gene: piece of DNA that codes for a protein. Allele: different forms of a gene.
Genotype: The allele combination of an individual.
Phenotype: The visible characteristics that result from a genotype.
Useful Terms
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I. Human ChromosomesA. Types
1. Sex Chromosomes – 1 pair – carry the genes that determine male and female features (also some non-sex traits)
1. X and Y do not look alike but behave as a homologous pair at meiosis
2. XX = female, XY = male2. Autosomes – non-sex chromosomes (22
pairs) – genes are unrelated to sex determination
Chromosomes and Human Genetics
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PUNNETT SQUAREMethod for determining possible allele combination for the offspring
Gametes outside
Offspring Inside
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Determining Sex…X X
X
Y
MOM
DAD
X X X X
X Y X Y
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TallP Dwarfx
F1 All Tall
Phenotype
One Example of Mendel’s Work
Clearly Tall is Inherited…What happened to Dwarf?
F1 x F1 = F2
F23/4 Tall1/4 DwarfDwarf is not missing…just masked as “recessive” in a diploid state… there IS a female contribution.
1. Tall is dominant to Dwarf
2. Use D/d rather than T/t for symbolic logic
DD dd
Dd
GenotypeHomozygous
DominantHomozygous
Recessive
Heterozygous
Dwarfdd
TallDdd
TallDd
TallDDD
dDPunnett Square:
possible gametes
possible gametes
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In hamsters, white fur color (W) is dominant to brown fur color (w). If you cross a heterozygous female with white fur color (Ww) with a male that has brown fur (ww), what genotypes and phenotypes would you see and in what ratios?
Sample Problem
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Genetics After MendelRed
xYellow
All OrangeWhen these alleles go walking, they both do some talking (codominance)!
OK, so we cannot use R/r nor Y/y so we pick a third letter…P for the petal color gene.
Notice: we do NOT mix R/Y or r/y!
PRPR PYPY
PRPY
F1 x F1 = F2
F2YellowPYPY
Orange
PRPY
PY
Orange
PRPY
RedPRPRPR
PYPRPunnett Square:
possible gametes
possible gametes
P
F1
This F2 will NOT have a 3:1 ratio of phenotypes.
Instead it shows a 1:2:1 ratio!
The exception here proves the rule.
After 1900 several scientists tried to replicate Mendel’s crosses using other species including snapdragon.
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In horses, black and white coat colors are codominant. Heterozygous horses have gray coats.
Black horse genotype: HBHB
White horse genotype: HWHW
Gray horse genotype: HBHW
What would be the possible genotypes and phenotypes of the following crosses?
Black x WhiteGray x Gray
Sample Problem
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Blood Types
Blood Type:ABABO
Genotype IAIA , IAi IBIB , IBi IAIB ii
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1. If a male is homozygous for blood type B and a female is heterozygous for blood type A, what are the possible blood types in the offspring?
Try These
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2) Is it possible for a child with Type O blood to be born to a mother who is type AB? Why or why not?
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Chromosomes and Human Genetics
I. Human ChromosomesA. Types
1. Sex Chromosomes – 1 pair – carry the genes that determine male and female features (also some non-sex traits)
1. X and Y do not look alike but behave as a homologous pair at meiosis
2. XX = female, XY = male2. Autosomes – non-sex chromosomes (22
pairs) – genes are unrelated to sex determination
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B. Determining Sex…X X
X
Y
MOM
DAD
X X X X
X Y X Y
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II. Gene LocationA. Linked – Linkage Groups – genes located so close
together on a chromosome that the traits always seem to appear together
Ex. Red hair and frecklesEx. Colorblindness and Hemophilia
X X
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B. Sex-linked Traits – genes on the sex chromosomes
- Expression of certain genes often appears more in one sex than the other
- Males require only one copy of a gene since they only have one X chromosome
- See Royal Families of Europe Pedigree- Ex. Eye color in fruit flies, hemophilia, color-blindness
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Colorblindness Tests
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X-Linked/Sex Linkage – do not write
Genes present on the X chromosome exhibit unique patterns of inheritance due to the presence of only one X chromosome in males.
X-linked disorders show up rarely in females X linked disorders show up in males whose
mothers were carriers (heterozygotes)
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Practice Sex-linked Problems…. What will the result of mating between a normal
(non-carrier) female and a hemophiliac male?
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A female carrier who is heterozygous for the recessive, sex-linked trait causing red-green colorblindness, marries a normal male. What proportions of their MALE progeny will have red-green colorblindness?
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Hemophilia is inherited as an X-linked recessive. A woman has a brother with this defect and a mother and father who are phenotypically normal. What is the probability that this woman will be a carrier if she herself is phenotypically normal?
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C. Gene Interactions
1. Polygenetic trait – many genes influence a single trait (ex. Height, intelligence)
2. Pleiotropic effect – one gene having many effects (ex. Gene to make testosterone)
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Pleiotropy Expression of a single gene has
multiple phenotypic effects Marfan Syndrome – abnormal
gene that makes fibrillin (important in connective tissues)
?
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III. Genetic Analysis
A. Karyotype – visualized chromosomes stained, squashed, and photographed at metaphase
- They are characteristic of the species or individual
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B. Pedigree – chart showing family relationships (see
worksheet)
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Pedigree Analysis Method of tracking a
trait through generations within a family.
Good method of tracking sex-linked traits as well as autosomal traits.
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Sex-Linked Pedigree Shows gender bias
with males exhibiting the trait more often than females
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Autosomal Dominant Pedigree
Autosomal dominant traits do not skip a generation
Autosomal dominant traits do not show gender bias
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Autosomal Recessive Pedigree
Autosomal recessive traits skip a generation
Autosomal recessive traits do not show gender bias
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IV. Non-Mendelian Genetics
1. Incomplete Dominance – blended inheritance
- Neither form of the gene is able to mask the other
- Ex. Snap dragon petal color- R1R1 – RED- R1R2 – PINK- R2R2 - WHITE
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Incomplete Dominance
Neither allele is dominant Heterozygotes are a blend of
homozygous phenotypes = no distinct expression of either allele
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Try these In a plant species, if the B1 allele (blue flowers)
and the B2 allele (white flowers) are incompletely dominant (B1 B2 is light blue), what offspring ratio is expected in a cross between a blue-flowered plant and a white-flowered plant?
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•What would be the phenotypic ratio of the flowers produced by a cross between two light blue flowers?
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2. Codominance
No dominance and both alleles are completely expressed
Ex. Cat color C1C1 – Tan C1C2 – Tabby (black and tan spotted) C2C2 - Black
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Try These1. Cattle can be red (RR = all red hairs), white
(WW = all white hairs), or roan (RW = red & white hairs together.
a. Predict the phenotypic ratios of offspring when a homozygous white cow is crossed with a roan bull.
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b. What should the genotypes & phenotypes for parent cattle be if a farmer wanted only cattle with red fur?
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1. A cross between a black cat & a tan cat produces a tabby pattern (black & tan fur together).
a. What pattern of inheritance does this illustrate?
b. What percent of kittens would have tan fur if a tabby cat is crossed with a black cat?
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3. Multiple Alleles More than 2 alleles for one trait Ex. Eye color, hair color, blood type, guinea pig fur
color ABO blood groups
Each individual is A, B, AB, or O phenotype Phenotype controlled by marker on RBC IA and IB alleles are dominant to the i allele IA and IB alleles are codominant to each other
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Blood TypesBlood Type:ABABO
Genotype IAIA , IAi IBIB , IBi IAIB ii
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Try These1. If a male is homozygous for blood type B and a female
is heterozygous for blood type A, what are the possible blood types in the offspring?
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2) Is it possible for a child with Type O blood to be born to a mother who is type AB? Why or why not?
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3. A child is type AB. His biological mother is also type AB. What are the possible phenotypes of his biological father?
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Human hair color follows a similar pattern:Alleles: HBn = brown HBd = blonde hR = red hbk = black
HBnHBn = dark brownHBnHBd = sandy brownHBnhR = auburnHBnhbk = dark brown
HBdHBd = blondeHBdhR = strawberry blondeHBdhbk = blonde
hRhR = redhRhbk = red
hbkhbk = blackDominant does NOT mean frequent! Recessive can
be common!